Is the price too high for this character?
As of June 2003 it has become, under a new EC directive, illegal to use timber impregnated with creosote in certain situations where frequent skin contact might occur. This is due to new studies which assert that creosote has a greater concentration of cancer producing toxins than previously thought.
I think that the use of such things as railway sleepers in private gardens might well remain one of those "gray areas", although it is beyond my abilities to interpret the law. It certainly seems as if they are still available on the market in 2004.
To help you decide, I have posted here a link to the DTI's FAQ on the subject, also an article from the telegraph.
I have found that pressure-washing sleepers during construction, using a detergent brings the surface to a nice, clean appearance (bear in mind where the run-off goes). I have never noticed any detrimental effect from sleepers on plants or skin, but I would not now build seats, for example, using them.
There are also issues of disposal and use of non-recycled timber to balance against the small health risk of using sleepers. They also have far more character to them, which is part of the appeal. It is, however, for each individual to weigh up the issue and decide for themselves whether or not to use them.